The invention relates to radiation shielding for drywell containment electric penetration assemblies in nuclear reactor facilities.
A stationary nuclear power generating station is designed and used to sustain nuclear fission in a self-supporting chain reaction, thereby providing a source for the production of a large amount of gamma, neutron, and other particulate and electromagnetic radiation. Should there occur an accident involving loss of coolant, an even higher level of radiation will result. This radiation is potentially harmful and heavy shielding around the interior of the reactor consequently is required to protect operating personnel and equipment in the vicinity of the facility from being exposed to excessive and possibly harmful doses of radiation. In monitoring the activity of the core of uranium or other fissionable material located inside the reactor, it is necessary for instrumentation leads or conductors to be brought out from the radioactive area through the wells of the nuclear reactor face. The leads serve to connect an instrument such as a thermocouple within the radioactive area to a device such as a potentiometer located outside the containment wall of the nuclear generator facility. A drywell penetrator assembly conventionally is used to pass the electric conductors or cables through an opening in the well interfacing the nuclear reactor with those interior areas of the facility to which authorized persons are permitted access. For a safe level of radiation leakage to be maintained, it is essential therefore that the penetration assembly itself not constitute a channel or window through which the harmful radiant energy can possibly escape either directly or by scattering. Additional shielding consequently must be provided both inside and about the penetration assembly in order to attain a level of protection that does not destroy the effectiveness of the uninterrupted portion of the protective shielding wall that has been constructed around the contaminated room.
Radioactivity present within a nuclear reactor installation can be classified into at least four categories of radiation and subatomic particle emissions, each having different characteristics. The four types of radioactivity are alpha particles, beta particles, gamma rays and neutrons. It has been found that relatively simple radiation shields with individual thicknesses of but a few hundredths of an inch effectively prevent leakage of alpha and beta particles. In contrast, gamma rays and neutron emissions possess tremendous penetrating power and can be attenuated or controlled only by inserting appreciable layers of various appropriate materials within the containment shield. Although no single shielding material in the prior art has proved capable of attenuating or extinguishing all four types of radioactivity at the same time, any arrangement or combination of shielding materials which can control and absorb gamma rays and neutrons is generally regarded sufficient to extinguish alpha and beta particles. Therefore, consideration of the latter two categories of particle radiation usually may be discounted when confronting the problem of designing an effective protective radiation casing around the instrumentation leads.
In the prior art, the cables or pipes containing the instrumentation leads extending from the contaminated interior of the reactor were necessarily bent into assorted non-rectilinear shapes in an effort to prevent the linear passage of radiation through the containment wall. Thus in a few instances, cables would first be formed into an S-shape, then fed through the drywell containment wall and finally sealed appropriately by injecting suitable thermoplastic or thermosetting materials around the cables in an effort to prevent the scattering of neutron and gamma radiation. Because of the sealing around the cables, however, it was not possible to monitor the potential gas leakage rate seeping through the interfaces either between the cables and the molded materials or between the copper conductors and the cable-insulating materials. It was believed therefore that the continuous expansion and contraction of the molded materials due to the heating and cooling of the adjacent cables when the reactor was in operation contributed to a potential source of radiation leakage in the prior art. Furthermore, in an effort to maximize the protection against harmful radiation, prior art penetration assemblies frequently would have to be filled completely, albeit uneconomically, throughout their lengths by layers of material suitably capable of controlling the neutron and gamma ray emissions. The employment of this procedure, however, contributed to increased overheating of the cables embedded inside the penetration nozzle.